Method of making surface-type and point-type rectifiers and crystalamplifier layers from elements



Feb. 1, 19,55 K SE|LER 2,701,216

METHOD OF MAKING SURFACEI-TYPE AND POINT-TYPE RECTIFIERS ANDCRYSTAL-AMPLIFIER LAYERS FROM ELEMENTS Filed April 5. 1950 (ar/1 Gemor-nSI1 74) .Sftl H1861: mpemu/'ef A I ,n l 2 DB reacfimzane l -ll PUMP 8Cl, 42!

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mem] mefu/ ydlf) Mn) v Maly/$8 1 mary/'name2 mlnimiz/'ngzane UnitedStates Patent O METHOD F MAKING SURFACE-TYPE AND POINT-TYPE RECTIFIERSAND CRYSTAL- AMPLIFIER LAYERS FROM ELEMENTS Karl Seiler, Numberg,Germany, assignor to International Standard Electric Corporation, NewYork, N. Y., a corporation of Delaware Application April 5, 1950, SerialNo. 154,064

Claims priority, application Germany April 6, 1949 4 Claims. (Cl.117-106) Unlike the large variety of point-type rectifiers (crystaldiodes) there are only a few combinations of metals and semiconductorswhich exhibit a surface-type rectifying effect. Schottkys analysis ofsemiconductor rectification (see f. i. Zeitschrift fr Physik, vol. 118,1942, pp. 539-592) sets forth several reasons which may serve to clarifythis fact.

(1) The relative impurity concentration throughout the semiconductorshould be a minimum in the zone of increased resistance immediatelyadjacent the metallic electrode, in order to avoid short-circuits. Anyincrease in the number of impurities in this zone means greatprobability of short-circuits due to so-called by-passes.

(2) In the adjacent semi-conducting zone, however, adequate conductivityhas to be provided by lattice-disturbing (impurity) atoms sufiicient innumber to greatly accentuate the zone mentioned in paragraph (l) wherethe so-called borderline effects take place.

To fulfil this requirement the disassociation level of the impurityatoms should be low (the impurity atoms should not diffuse readily),while the mobility of the electric carriers should be high. That only afew surface-type rectifications effects have been realized so far,derives from the fact that these two demands have not, or have hardly,been possible of fulfilment so far, for the known methods of makingspecified layer patterns are based exclusively on such processes wherethe space distribution of the impurities, is only obtained bymodifications introduced by a secondary process by removing impuritiesat a suitable temperature, by chemical reaction (f. i. dissolving,lacquer-layers applied later on, etc.). Hence in making some specifiedlayer pattern, the substances intended to produce the desired effectsare placed at their respective locations in a first operation where noattention can be paid as to obtaining any specified space distributionof impurities, while that specified distribution, essential for properperformance, is brought about only subsequently in a second processgoverned by laws different from those of the rst one. Thus one has beenforced to simply accept as given whatever impurity concentration anddistribution resulted from that second process without the possibilityof introducing such corrective action of ones own as might be desirableto secure optimum distribution of impurities across the layer. Withregard to the fact that very delicate arrangements are involved withlayers present only as extremely thin films, it is obvious that theprobability of inadequate results is rather large if these two essentialprocesses are practiced one after the other.

In rectiiiers involving compound-type semiconductors (in particularcuprous oxide) this fact is particularly pronounced as here the changein impurity concentration is caused by a chemical reaction which in turnis to introduce the stoichiometric unbalance of the semiconductorcompound.

Generally a high impurity concentration is provided in the zones of thesemiconductor more remote from the metallic electrode. But a certainmaximum impurity concentration should not be exceeded in the zone of thesemiconductor immediately adjacent the metallic electrode. This meansthat along the main direction of current ow the impurity concentrationwill have to obey a specified functional law, i. e. the dependence ofthe impurity concentration at a particular layer level will de- 8 pendon the distance between that layer and the metallic 2,701,216 PatentedFeb. 1, 1955 ice electrode. This distance, however, is extremely smallso it is obvious that rather erratic results are liable to occur in thefew processes available for controlling the impurity concentration.

The drawbacks of the known methods are avoided in this invention byproviding that the ultimate structure of the layer pattern is no longerobtained from two independent individual processes, but that the basicand the impurity substances are deposited simultaneously on the providedfoundation, and that the specified relative impurity concentration liscontrolled to lit the needs of each particular layer while the layer isbeing built, so thje desired space distribution of impurities is broughta out.

It is known that the elements boron, silicon, germanium, and telluriumhave semiconducting properties. In view of the delicacy of the processesto be controlled, the degree of chemical purity of the substances coatedon the foundation electrode is of outstanding importance in makingrectifiers.

In order to achieve uniform results and to control the porportion of theimpurity admixture, it might seem desirable to vaporize thesemiconductor materials and the impurity substances and deposit themfrom the vaporous state onto a suitable foundation. However, due to thehigh vaporizing temperatures of the materials involved there is inducedthe hazards of chemical reactions of the basic material or even theimpurity contents with the material of the Crucible which would produceuncontrolled and undesirable etfects on the rectifying properties.

In order to avoid such high temperatures in production, one starts outin the known way from a liquid compound of the elements involved whichby chemical rectification and customary chemical purifying methods canbe prepared with a very high degree of purity and which in turn may bereduced chemically under proper conditions upon reaction with someequally pure reducing agent. Simultaneously with this reduction, theimpurity substance may be treated in the same manner. With therelatively ample choice present in selecting admixtures tosemiconducting elements, there is usually found some suitable chemicalreaction mechanism which enables simultaneously synthesizing theimpurity admixture. In a functional schematic, the nature of the processto be adopted is indicated in Fig. l. A, B, C, and D refer schematicallyto devices where the substances needed for assembling the rectifyinglayer are obtained in their purest form by available processes accordingto what has been described above. As indicated schematically, thesesubstances are then fed through pipes La, Lb, Lc, and Ld to the device Ewhere the layer pattern is formed. In Fig. l, these pipes La and Lbdirectly feed the device E with no prior intermixing of the substancesbeing possible. Of course, one, two, or even all of the pipes could bejoined in a common duct whenever intermixing of the substances isdesired or permissible at such a relatively early stage of theproduction process. Equally in a schematic manner are shown controlmechanisms Ra, Rb, Rc, and Rd at some arbitrary point along the ductswhich allow any desired decrease or increase in the feeding rate of anyof the ingredients while the semiconducting system is being built up. Itis by no means essential that these controlling devices be insertedsomewhere between the units A and E, B and E, etc., these controllingfacilities may as well be incorporated straight in the units A, B, C, Dor any additional units that might be present. Care will have to betaken, however, to prevent undesired reactions on the other devices B,C, D whenever at the junction of the pipes or individual pipe ducts thegas or Vapor rate of device A is increased.

To practice this method, the elements boron, silicon, germanium, ortellurium may be used because of the high mobility of their electriccarriers. To provide optimum efficiency, the impurity concentration atthe back or counterelectrode is chosen large enough to prevent virtuallyany barrier layer from forming there.

As an example of the preparation of the semiconducting substance inbuilding of a layer pattern, silicon may be reduced with hydrogen fromsilicon, tetrachloride or with zinc, in order to obtain pure silicon.

right, respectively, of silicon in the periodic table of elements act asacceptors anddonors, respectively, in the semiconductor. To synthetize asurface-type rectifying silicon layer, the basic material (silicon) andthe impurity admixture (for example boron) are reduced simultaneouslyunder the equations The production setup will have about the appearanceoutlined in Fig. 2. In the reaction oven O, the reaction productdeposits on a conducting foundation, (for example, carbon, or otherhigh-melting material not alloying with the basic material) or on someinsulating foundation (aluminum oxide, or similar substances).

As boron is used in very low concentrations only, one preferably uses amixture of boron chloride and silicon tetrachloride in the second feedercurrent.

Silicon may be provided with a small percentage of tin or germanium byadding SnCl4 or GeCl4 to the ow of SiCl4.

Other methods of introducing into the reaction the impurity material mayconsist for example of thermal decomposition out of a suitable compoundor in adding it as an element to the outgoing ow.

The essential feature is that the impurity concentration is open to anyprogrammed control during the buildingup of the semi-conducting layer.

In a similar way one may make surface-type rectifying discs fromgermanium, by reducing germanium tetrachloride along with tintetrachloride or silicon tetrachloride or boron chloride in the presenceof hydrogen, or by adding arsenic or antimonium to the current in theshape of a compound of these elements, with hydrogen.

The reducing agent will he so chosen that small amounts of it whendissolved, in the semiconductor will not result in any marked degree ofconductivity, or that if such is the case -they will cause if possiblethe kind of conductivity that is wanted anyhow when adding the impurity.Hydrogen is a reducing agent of relatively very neutral character.

In case some metal having relatively high vapor pressure is beingreduced, such as zinc, one may operate without a ilow. In Fig. 3, Orefers to an electrically-operated tubular oven with two separatewindings in series-connection. In the rear section, zinc vapor isgenerated which ows in the opposite direction to the mixture of silicontetrachloride and boron chloride, or silicon tetrachloride and germaniumtetrachloride, the fractional composition of which may be varied anytime. In the reaction zone are again the bases on which thesemiconducting material deposits. These mentioned methods may beextended to boron as well.

Another example is illustrated in Fig. 4, which however shows only thepart indicated by E in Fig. l. In the inner tubing Ri, closed at oneend--when required it may also be open at either end-there is fed fromone side simultaneously some halogen compound or halogen compounds ofthe basic material as well as of the impurity material at suitable rateswhich undergo control during the reaction process as desired, and theyare brought to reaction with some material reacting with either halogen(i. e. with that of the basic material as well as with that of theimpurity material) so they deposit on bases placed in the reaction zoneat a point where reducing material is not, or almost entirely not,deposited.

Thus pipe Rl may for instance be entered from the left by silicontetrachloride and boron chloride while at the right aluminum isvaporized, so that from the right aluminum vapor or low-order aluminumhalogens ows in a current opposing that of the aforenamed compounds. Inthe reaction zone, aluminum chloride is formed which has to flow leftthrough the open end of the tube in a countercurrent to the enteringsubstances. The carriers or bases are then placed in such a section ofthe reaction zone where no aluminum deposits as in this process it isboron that has to serve as the impurity admixture. If required, even tinor germanium may be deposited as impurity substances.

Tube R1 is surrounded by some outer shell Ra to which the vacuum pumpconnects at the right. The entire assembly is accommodated in an oven(not shown).

As obviously'the halogens entering the tube and those leaving it passeach other in opposite directions, it is convenient to direct theseopposite ows by appropriate partitions. One may thus for example,partition the tube up to the reaction zone by a horizontal wall to feedsilicon halogen and boron halogen in the lower half While the upper halfserves for the return of the formed aluminum chlorides. Again, one maytake to a concentric subdivision of tube R up to the reaction zone, inorder to feed through the inner tubeA silicon tetrachloride and boronchloride, and to remove through the concentric ring space the halogensproduced in the reaction. But one may even avoid this opposite currentpattern by designing all of the rectier-making process as a like-currentoperation, with all of the agents participating in the reactions passingthrough the tube left or right in the same direction of flow. In thiscase, and in the selected example, aluminum or low-order chlorides ofaluminum would be fed from the left to the reaction zone in a vaporizedstate through an extra duct, while all of the ingredients and thosesubstances as are set up leave to the right with of course thedepositing pure basic and impurity substances settling out somewhere ata suitable place on appropriate deposit-carriers. With the describedprocess, one not only can expect uniformity of the output material, butthere is further facilities for making rectiers with optimum propertiesby varying the percentages of the mixing basic and impurity substances.

In addition to aluminum, as a reducing agent there may be used zinc,hydrogen, etc. With aluminum as a reducing agent, the drawback isencountered that tube Rr in Fig. 4, which is conveniently made ofquartz, is corroded by aluminum so it becomes useless before long. Thiseven introduces changes in the reaction conditions which even may leadto a displacement of the reaction zone. To avoid this, the aluminum isconveniently inserted in a short tubular socket of sinteredcorundum-aluminum oxide--so the quartz tubing is protected.

The process of the present invention permits making surface-typerectiers with entirely symmetrical electrical characteristics (so-calledlimiters), if the local impurity concentration is held low not at theoutside (i. e. the interface to an adjacent electrode) but in thecenter-layer of the semi-conductor. Fig. 5 shows the overall structuralpattern of such a limiter where the impurity concentration is minimizedin the center-layer over a layer thickness d. The thickness of themarginal zone bordering at the low-concentration zone is some lO-7centimeters, or a few atoms across.

The effect of the thickness d of the low-concentration zone on theproperties of the limiter, i. e. on the diffusion voltage level Vd(cutoff-voltage) is shown in Fig. 6. The current J has been plottedversus the voltage, and this for two different values of a'. Herewith,1(2) exceeds d0), so even Va@ exceeds Vdl).

Particularly interesting is a layer pattern which results in atransistor effect. To this end it is essen-tial that the type ofelectric carriers changes right under the surface. Electron-conductinggermanium crystals (n-type) which at the surface are modified forhole-conduction (p-type) have been described. This structure gives riseto a barrier layer at the interface of the n-type and p-type conductinggermanium layers so the current of the emitter electrode is forcedradially into the surface. It is only then that the barrier layer underthe collector is so affected that the known power-amplifying control ofsame takes place.

In making such a transistor, known techniques start from solid n-typeconducting germanium and then the surface is made p-type conducting bysome particular treatment thereof. Such surface treatment obeys laws ofits own, so only in the most exceptional cases will it perform theconversion from the n-type to the p-type in the Way required for optimumtransistor operation. Hence, it is advantageous as provided in thisinvention to assemble a transistor from its basic elements so that onthe foundation there is rst applied the layer of the basic material(such as germanium), along with a donor causing electron conduction,with the change in the type of conduction subsequently brought about inthat the donor, at a specified stage of the layer-assembly, is replacedby an acceptor, which causes hole-conduction in the basic material whenit is deposited along with the latter. One also may begin by making ap-type conducting foundation to coat it subsequently with a thin n-typeconducting lm.

I claim:

1. In the method of making semi-conductors each having a plurality ofdiierent conductivity zones therein, the

steps of simultaneously depositing from the vapor state onto a basematerial a semi-conducting substance commingled with an impuritysubstance selected from a group consisting of donor and acceptorimpurities, and varying the proportion of said substances while applyingthem to the base in the form of a. composite layer of said substancesgraduated in its cross-section as between amounts of the two substancesapplied.

2. Method according to claim 1, in which the semiconducting substancesare chosen from the group consisting of boron, silicon, germanium andtellurium.

3. Method according to claim 1, in which the highest amount of impuritysubstance is applied adjacent the base material.

4. Method according to claim 1, in which the lowest amount of impuritysubstance is placed in the center portion of the layer.

References Cited in the ile of this patent UNITED STATES PATENTS VanArkel Aug. 26, 1930 Hyde June 26, 1934 Prescott Oct. 8, 1940 WaltherMar. 9, 1943 Sanlaw Nov. 28, 1944 Essig Apr. 19, 1949 Martin Oct. 11,1949 Henderson et al Mar. 2l, 1950 Fisher et al May 15, 1951 Teal June12, 1951

1. IN THE METHOD OF MAKING SEMI-CONDUCTORS EACH HAVING A PLURALITY OFDIFFERENT CONDUCTIVITY ZONES THEREIN, THE STEPS OF SIMULTANEOUSLYDEPOSITING FROM THE VAPOR STATE ONTO A BASE MATERIAL A SEMI-CONDUCTINGSUBSTANCE COMMINGLED WITH AN IMPURITY SUBSTANCE SELECTED FROM A GROUPCONSISTING OF DONOR AND ACCEPTOR IMPURITIES, AND VARYING THE PROPORTIONOF SAID SUBSTANCES WHILE APPLYING THEM TO THE BASE IN THE FORM OF ACOMPOSITE LAYER OF SAID SUBSTANCES GRADUATED IN ITS CROSS-SECTION ASBETWEEN AMOUNTS OF THE TWO SUBSTANCES APPLIED.